This invention relates to a tensioner used for applying tension to a timing belt, a timing chain or the like for a vehicle engine. The invention relates more specifically to a tensioner with a projection-preventing hook for its plunger.
Tensioner have been used widely to maintain proper tension, and to prevent vibration, in timing belts and timing chains that connect the crankshaft of an internal combustion engine to its camshaft or camshafts. In a typical engine timing chain or belt, a tensioner lever is pivotally supported on the engine body (see Japanese utility model publication No. Sho 58-38189). The lever is maintained in sliding contact with the slack side of the timing belt or chain in order to apply tension to the chain. A tensioner exerts a force on a back surface of the lever by means of a plunger projecting from a tensioner housing.
FIG. 7 shows a conventional tensioner 20 mounted on an engine block 30. The tensioner 20 has a housing 22, and a plunger 21 spring-urged in the projecting direction and having a head portion 21a. A lock pin 23 is provided on a side wall of the plunger 21 adjacent the front end of the plunger, and a fulcrum pin 24 is provided on a side wall of the housing. A projection-preventing hook 25, which can move into locking engagement with the lock pin 23 when the plunger is withdrawn, is pivotally supported on the fulcrum pin 24.
The projection-preventing hook holds the plunger 21 in a withdrawn position when the tensioner 20 is being handled prior to mounting on the engine block 30.
As shown in FIG. 7, the tensioner is mounted so that the notch of hook 25 faces upward. An inclined portion 25a of the hook 25 is engaged by the lock pin 23. After the tensioner is mounted on the engine block 30, when the plunger 21 is moved slightly in the withdrawing direction, by a force exerted on its head portion 21a, inclined portion 25a serves as a cam. The force exerted on inclined portion 25a by the lock pin 23 causes the hook 25 to rotate on the fulcrum pin 24, whereupon the hook disengages from the lock pin 23 and hangs by its own weight. With the lock pin 23 thus disengaged the plunger is released, and the tensioner 20 can function in the normal manner.
Since the projection-preventing hook 25 is pivoted on the fulcrum pin 24, it is possible for the hook 25 to return to a locking condition due to continuous slight vibrations during operation of the engine. It is also possible for the projection-preventing hook 25, which is freely pivoted on the fulcrum pin, to swing like a pendulum due to continuous vibration of the engine, and to generate additional vibration noises. Moreover, the vibration may loosen the mounting of the hook 25, allowing it to fall off the fulcrum pin 24, causing a breakdown in the engine.
The projection-preventing hook 25 may be mounted on the engine block 30 with its notch facing downward, i.e., in a direction opposite to the usual mounting direction shown in FIG. 7. The condition in which the notch faces downward is illustrated in FIG. 8, in which a tensioner is shown mounted on an engine block 30 by threads, and its rotation is stopped when a flange 27 engages the engine block. In the case depicted in FIG. 8, where the notch of the projection-preventing hook 25 faces downward, the operation of the hook is as depicted in FIGS. 9(a) and 9(b). When the plunger 21 is moved slightly in the withdrawing direction by an external force F applied to the end of the plunger, the projection-preventing hook 25 is raised as shown in FIG. 9(a), and the locking relationship between the projection-preventing hook 25 and the lock pin 23 is temporarily released. However, the projection-preventing hook 25 may return to locking engagement with the lock pin 23, as shown in FIG. 9(b), by pivoting under its own weight, or as a result of engine vibration. Thus, there was a problem that the projection-preventing hook 25 and the lock pin 23 could not be disengaged irreversibly, and the conventional tensioner would not function reliably.
Although a thread-mounted tensioner such as shown in FIG. 8 can be mounted and demounted easily during engine assembly and maintenance, the direction in which the notch of its projection-preventing hook faces cannot be predetermined easily. Accordingly, although the thread-mounted tensioner has significant advantages, demand for thread mounted tensioners has been low.
A general object of the invention is to provide a tensioner having a projection-preventing hook, that can reliably function as a tensioner irrespective of the position in which the tensioner is mounted on an engine block. Another object of the invention is to prevent vibration noise due to the projection-preventing hook.
The tensioner in accordance with the invention comprises a tensioner housing having front and rear ends. A plunger extends outward from the interior of the housing at the front end thereof, and has a forward end outside the housing. The plunger is slidable into and out of the housing along an axis from a withdrawn position to an projected position, and is spring-urged in a projecting direction along the axis. A lock pin, provided on the plunger adjacent the forward end of the plunger, extends laterally with respect to the axis. A fulcrum pin is provided on a side wall of the tensioner, and a projection-preventing hook is pivoted on the fulcrum pin. The fulcrum pin is positioned so that the hook can engage the lock pin when the plunger is in its withdrawn position. A resisting member, fitted onto the fulcrum pin, exerts a frictional force on the hook to produce resistance to pivoting movement of the hook.
In one preferred embodiment of the invention, the resisting member is an O-shaped ring. In another preferred embodiment, the resisting member is a plate-shaped spring. The invention is particularly advantageous where the tensioner is mounted by mounting threads provided on a portion of the housing adjacent the rear end thereof, so that the rotational position of the housing, and the orientation of the hook, are not predeterminable.
A first advantage of the invention is that, regardless of position at which the tensioner is mounted on an engine block, regardless of the direction of the pivot axis, and regardless of whether the hook is oriented with its opening facing upward, downward, or in an intermediate direction between the upward and downward directions, pivoting movement of the projection-preventing hook can be suppressed. Therefore, even if the projection-preventing hook is oriented so that its own weight would tend to move it to a latching position where it would engage the lock pin on the plunger, and even though it is subject to continuous slight vibration during operation of the engine, it does not return unexpectedly to its latching position. Thus, the tensioner of the invention is more reliable than conventional tensioners having projection-preventing hooks. Furthermore, when the projection-preventing hook is engaged with the lock pin, the tensioner of the invention reliably retains the plunger in its withdrawn position unless a manual disengaging force is applied to the projection-preventing hook.
Since the resisting member absorbs continuous slight vibrations during operation of the engine, which would otherwise be transmitted to the projection-preventing hook, vibration noise, which would otherwise be generated by vibration of the hook, is suppressed.
Where the resisting member is an O-shaped ring, sliding friction acts between the ring and the projection-preventing hook. Thus, pivoting of unlocked projection-preventing hook is suppressed, the locking pin is released reliably, and the released condition is reliably maintained.
Where the resisting member is a plate-shaped spring, the resisting member exerts pressure on the projection-preventing hook, positively preventing pivoting of the projection-preventing hook even during continuous slight vibrations caused by operation of the engine. Moreover, the projection-preventing hook reliably releases the lock pin, and the released condition is maintained reliably.
When the tensioner is provided with threads for engagement with a housing on an engine block or the like, and rotation of the tensioner is stopped by engagement of a flange on the tensioner with a part of the housing, the rotational position of the tensioner cannot be predetermined reliably. However, because the resisting member acts to resist rotation of the hook about its pivot, the hook operates properly regardless of the rotational position of the tensioner.